1. Field of the Invention
The invention relates to synchro devices particularly with respect to apparatus for demodulating synchro output signals into corresponding d.c. signals.
2. Description of the Prior Art
Three-wire synchro devices are commonly utilized for sensing the angular position of members. The rotor of the synchro device is coupled to the member with an a.c. excitation signal applied to the excitation winding of the synchro device. The excitation signal is coupled from the excitation winding to the output windings of the synchro device to provide three a.c. signals amplitude modulated in accordance with the angular position of the rotor and either in phase or out of phase with the excitation signal in accordance with the rotor position. In typical applications, the three-wire signals from the synchro device are converted to two-wire a.c. signals proportional to the sine and cosine of the rotor angle via a Scott-T transformer. The sine and cosine outputs from the Scott-T transformer are conventionally converted into d.c. sine and cosine signals via respective synchronous demodulators connected to the sine and cosine outputs of the transformer. In digital systems the d.c. sine and cosine signals may be converted into digital format via an analog-to-digital converter and applied to a microprocessor for appropriate computations.
This arrangement suffers from the disadvantage that gain discrepancies between the sine and cosine channels of the apparatus result in errors in the ultimate angular output rendering the apparatus commercially unacceptable. These gain discrepancies have heretofore only been alleviated by complex and tedious component matching with respect to the two channels or by undesirable trimming circuitry and procedures. Primarily the problem exists in the demodulators utilized in the apparatus. Ideally the demodulators should be identical with respect to each other but as a practical matter they are not, requiring extensive and costly calibration. Not only are the adjustments of the two channels to obtain identical gain with respect to each other tedious and undesirable, but the gains of the two channels tend to drift with time and temperature or other environmental factors, to reduce the device accuracy to unacceptable values. Additionally, Scott-T transformers tend to be bulky, heavy and expensive, particularly with respect to precision transformers wherein the sine and cosine channels are closely matched.
An alternative prior art arrangement may utilize the well known electronic Scott-T circuit in place of the Scott-T transformer. Such circuits also require precisely matched components with respect to the sine and cosine channels to provide the required accuracy and additionally require numerous expensive and bulky circuit elements for their implementation.
Thus it is appreciated that unless bulky and expensive circuitry and components were utilized that required precise matching and tedious trimming, the sine and cosine values applied to the digital processor would be so inaccurate as to result in commercially unacceptable accuracy in the ultimate angular result.
It is, therefore, a desideratum of the present invention to provide accurate angular data from synchro devices without the attendant extensive and bulky circuitry and components and the undesirable requirement for precisely matched channels or complicated and tedious trimming procedures and circuits.